This invention relates to a rotational vibratory viscometer for measuring viscosity of fluids at high temperature and high pressure; and is especially suited for, but not limited to, measuring the viscosity of high viscosity fluids such as molten polymers and bread dough.
Rotational vibratory viscometers are well known in the art, and generally comprise (i) a transducer having a tip immersible in a fluid the viscosity of which is to be determined, (ii) an electromagnetic drive coil for causing the tip to rotationally oscillate with a very small angular amplitude, (iii) a feedback control circuit for maintaining the angular amplitude of oscillation of the tip at a predetermined constant value irrespective of the viscosity of the fluid, and (iv) a circuit for determining the power supplied to the drive coil, usually by squaring the current supplied to said coil, which power is a measure of the viscosity of the fluid.
A viscometer of this type is described, for example, in "Viscometer for Energy Saving", J. V. Fitzgerald, F. J. Matusik and P. C. Scarna, Jr., Measurements & Control, April 1980 and in "Continuous Viscosity Control Improves Quality and Recovery" by Terri M. Walsh, Adhesive Age, December 1989. Similar viscometers are described in the references cited in said articles, as well as in U.S. Pat. Nos. 3,382,706; 3,710,614; 3,712,117; 3,762,429; 3,875,791; 4,299,199; 4,488,427; 4,566,181; 4,754,640; and 5,054,313.
The commercial use of vibratory viscometers of this type has been for the most part, in applications described in the foregoing references.
These prior art viscometers, however, are not suitable for applications utilizing high viscosity fluids, especially at high temperatures and pressures, such as the extrusion of plastic resins through dies to form various objects such as toys, wastebaskets and film.
Sensors are commercially available for measuring the temperature and pressure of resins in real time during the extrusion process; e.g. those manufactured by Dynisco, Four Commercial Street, Sharron, Ma. 02067 and Omega Engineering, Inc., One Omega Drive, Stamford, Conn. 06906.
However, no device for real time in-process measurement of high viscosity fluids at high temperatures and pressures has hitherto been available.
Because of the unavailability of such in-process high viscosity measurement equipment, it is necessary to monitor resin extrusion and similar manufacturing processes by obtaining a sample of the fluid and performing viscosity measurements on the sample. This is done by having part of the flow on its way through the extruder bypassed or extracted for measurement by one of the several commercially available viscometers especially designed for this purpose. Manufacturers of these on-line instruments include Seiscor Technologies, Tulsa, Ok. 74146; Rheometrics, Inc., Piscataway, N.J. 08854; and Gottfert Werkst Off-Prufmaschinen GmbH, Buchen, Germany.
A non-oscillating high viscosity viscometer.sup.1 with a channel for fixed plate measurement is reported by McGill University Professor John M. Dealy in U.S. Pat. No. 4,571,989. Dealy says there are many reasons for being able to determine rheological properties of the molten plastic during processing of that material. FNT True Shear Rheometer introduced May 1992, Interlaken Technology Corp., Eden Prarie, Minn. 55346.
The importance of knowing rheological properties such as fluid viscosity during the extrusion process cannot be overemphasized. The fact that many companies go to great expense to measure fluid viscosity on an offline basis bears out this contention. Measurement and control of the product directly in-line in real time while the process is occurring is very important in maintaining uniform quality of the extruded product. In an article in Food Extrusion Science and Technology, Marcel Dekker, Inc., N.Y. (1992) entitled "An On-Line Slit Rheometer for Measurement of Rheological Properties of Dough", C. G. Gogos and S. Bhakuni wrote the following regarding the current need for an extruder viscometer to overcome the shortcomings of measurement methods now in use:
"Obviously the viscosity function can be effectively used for monitoring product quality. In continuous food processing operations (e.g., dough extrusion), it can also be utilized for controlling the process. PA0 On the other hand, the growing interest in automating food processing operations has generated an immediate need for real-time knowledge of rheological properties so that one can maintain tighter control of process conditions and thus product quality. Most of the approaches to such measurements have utilized one of the following methods:
Placing pressure transducers before the die: complex die geometries render this method approximate at best. PA1 Using capillary or slit dies tapped on the processing equipment before the die: this allows a side waste dough stream to flow under the die pressure drop."
According to Dealy.sup.2 a variety of high viscosity rheometers are commercially available. However, most of them are off-line instruments. The time required to obtain a result from these rheometers can vary from minutes to hours, depending on the complexity of the measurement. A lot of product is extruded while such viscosity measurements are being made, since it is not practical to turn the extruder off while making the measurements. With such viscometers being off-line and/or requiring a considerable amount of time to make viscosity measurements, adjustment of the composition and/or process parameters of the fluid in an effort to keep its viscosity within the desired tolerance limits, is often fruitless. FNT .sup.2 U.S. Pat. NO. 4,571,989.
Off-line viscometers are very expensive. They require long signal delay. Efflux from the required side stream may constitute a waste problem. Major instrument components must be mounted near the process equipment.
Accordingly, an object of the present invention is to provide a transducer for a vibratory viscometer, having an improved rotary vibratory probe capable of directly and quickly performing inline measurements on high viscosity fluids at high temperature and pressure in real time; such performance not being hitherto within the capability of available transducers.